1. The Field of the Invention
The present invention relates generally to x-ray tube devices. In particular, embodiments of the present invention relate to analytical x-ray tubes employing a cathode assembly—target surface arrangement which contributes to reduced heat levels in various portions of the x-ray tube, and which allows the x-ray tube to be placed relatively closer to the sample to be analyzed, thereby improving the quality of the analysis that can be performed with the x-ray tube.
2. Prior State of the Art
X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. Such equipment is commonly used in applications such as diagnostic and therapeutic radiology, semiconductor manufacture and fabrication, materials testing, and sample analysis. While used in a number of different applications, the basic operation of x-ray tubes is similar. In general, x-rays, or x-ray radiation, are produced when electrons are produced, accelerated, and then impinged upon a material of a particular composition.
Regardless of the application in which they are employed, these devices typically include a number of common elements including a cathode, or electron source, and an anode situated within an evacuated enclosure. The anode includes a target surface which receives electrons emitted by the cathode. In operation, an electric current applied to a filament portion of the cathode causes electrons to be emitted from the filament by thermionic emission. The electrons thus emitted are accelerated towards a target surface of the anode under the influence of an electric potential established between the cathode and the anode. The interaction of these high energy electrons on the target surface causes x-rays to be emitted from the target surface.
The specific frequency of the x-rays produced depends in large part on the type of material used to form the anode target surface. Anode target surface materials with high atomic numbers (“Z” numbers), such as tungsten, are typically employed. The x-rays ultimately exit the x-ray tube through a window in the tube, and interact in or on various material samples or patients. As is well known, the x-rays can be used for sample analysis procedures, therapeutic treatment, or in medical diagnostic applications.
As discussed above, a portion of the electrons that impact the anode target surface convert some portion of their kinetic energy to x-rays. However, most of the kinetic energy does not produce usable x-rays, but is released in the form of heat. This heat, which can reach extremely high temperatures, is transferred throughout the anode and other x-ray tube structures, such as the window.
Some of the electrons simply rebound from the target surface and strike other “non target” surfaces within the x-ray tube. These are often referred to as “backscatter” or secondary electrons. These backscatter electrons retain a significant amount of kinetic energy after rebounding, and thus when they impact non-target surfaces such as the window, additional heat is generated.
The heat generated at the target surface as a result of normal x-ray tube operations, as well as the heat generated as a consequence of backscatter electron impacts, must be reliably and continuously removed. If left unchecked, such heat can compromise the performance of the x-ray tube, or damage it, and may ultimately shorten its operational life. As discussed below, the heat imparted to the window area is especially problematic in the context of analytical x-ray tubes (AXT).
In general, AXTs refer to a type of x-ray device that is typically used to produce a stream of x-rays that can be employed to facilitate, among other things, analysis and evaluation of material samples. Examples of analytical, evaluative, and control processes that can be performed or facilitated by AXTs include material composition analysis, fracture detection and evaluation, industrial material content control, thickness of material control and the like. AXTs possess a variety of useful characteristics that make them well suited for such applications. For example, AXTs are relatively compact and portable. Furthermore, AXTs produce results relatively quickly. Finally, the x-rays emitted by the AXTs are non-destructive to the sample being analyzed. This feature is particularly useful in that it facilitates, among other things, analysis/evaluation of materials in situ.
The operation of a typical AXT is relatively straightforward. Typically, a sample of material is bombarded with x-rays from the AXT. One or more detectors or other sensors placed near the sample are then used to capture, categorize, or otherwise sense the response of the sample material.
As is well known, different materials generally respond in different ways to the presence of x-rays. That is, as a consequence of variables such as chemical composition and structure, each material exhibits a characteristic response when struck by x-rays. Thus, when the beam of x-rays generated by the AXT is directed toward a sample of interest, the sample responds in a characteristic fashion that distinguishes it from other materials. Based upon the response exhibited by the sample, the user of the AXT is able to draw conclusions regarding the nature of the sample being analyzed.
One example of an analytical technique where AXTs are commonly employed is commonly known as x-ray fluorescence spectroscopy (“XRF”). In XRF applications, the sample is bombarded with a beam of x-rays from the AXT. The material responds by emitting characteristic x-rays which are received or sensed by the detector so as to facilitate evaluation of the sample.
It is generally the case that the quality of the results obtainable with an AXT improves as the distance between the target surface and the material sample decreases. This is at least partly due to the fact that a relatively shorter distance between the target surface and the sample translates to a relative increase in the number of x-rays striking the sample, and accordingly, an improved response from the sample. This consideration, among others, has lead to the development of AXTs having relatively compact geometrical arrangements.
In particular, the typical AXT employs a cathode disposed in close proximity to the target surface so as to maximize the electric field at the cathode, and hence the number of electrons striking the target surface, and thus, the x-ray flux produced by the device. As previously noted, the quality of sample analysis increases as a function of the proximity of the target to the sample. Thus, in an effort to improve the quality of analysis performed by AXTs, many design efforts have focused on attempting to compress the distance between the window and the target surface, as well as the distance between the sample and AXT window.
As noted earlier however, the cathode in a typical AXT is located near the target surface so as to maximize x-ray production by the device. Thus, as the target surface is moved closer to the window, the distance between the cathode and the window necessarily decreases as well. While such arrangements may enhance some aspects of the performance of the AXT, they have proven problematic for a variety of reasons.
One problem with such arrangements concerns electron bombardment of the window and surrounding structures. In particular, because the window is so close to the cathode, a large number of electrons emitted by the cathode inevitably strike the window, thereby imparting a significant amount of heat to the window and surrounding structures.
This problem has not gone unnoticed, and various attempts have been made to devise systems and structures to counteract the high heat levels typically present in the window area of known AXTs. However, while such systems and structures arguably provide a level of cooling in the window area, they also add to the overall complexity and bulk of the AXT. Additionally, the addition of window cooling devices and systems increases the overall cost of the AXT.
Another problem stemming from the proximity of the cathode to the window concerns the effects of the cathode filament material, typically tungsten, on the inner window surface. In particular, the high filament temperature required for electron emission causes a thin film of tungsten from the filament to be deposited on the inside window surface. The film of tungsten thus deposited blocks some x-rays from passing through the window and accordingly, a reduction of x-ray output through the window to the sample is realized. As is well known, the quality of analysis achievable with the AXT is at least partially a function of the x-ray output of the AXT. Accordingly, the reduction in x-ray output that stems from the formation of the tungsten film on the inner window surface acts to materially compromise the performance of the AXT.
Another problem posed by the tungsten film deposited on the inner window surface concerns the integrity of the characteristic response emitted by the sample undergoing analysis. In particular, because some of the x-rays produced by the AXT strike the tungsten film prior to impinging upon the sample, stray emissions are generated that contaminate and compromise the characteristic response emitted by the sample.
Finally, while efforts have been made to produce AXTs of relatively compact configuration, certain inherent features of components used in the x-ray generation process practically limit the extent to which such compactness may be achieved. In particular, the high voltages typically employed in AXT and other x-ray devices necessitate the maintenance of predetermined physical clearances between various structures such as the window, target and cathode. For example, structures that are at sufficiently different potentials, relative to each other, will cause arcing if they are placed too close together. At best, such arcing compromises the performance of the device, and may in some cases, cause a complete failure of the device. Thus, the high operational voltages serve to impose practical limits on the extent to which such structures can be moved more closely together.
Because of such limits, the overall compactness of the AXT is necessarily limited as well. As discussed above, the quality of analysis achievable with a particular AXT is at least partially a function of the distance between the target surface and the sample to be analyzed. Thus, the quality of the analysis performed is inherently limited by geometric arrangements which are, in turn, at least partially dictated by the high voltages typically employed by these devices.
One specific example of such geometric limitations concerns the nose portion of the x-ray tube evacuated housing where the window is located. Typically, the nose portion of AXTs is relatively wide so as to accommodate the spacing requirements imposed by the high voltages present in the device. In particular, the relative width of the nose permits the various components of the AXT to be positioned so as to avoid problems such as arcing. However, wide nose portions in many cases limit the usefulness of AXTs in confined spaces or close quarters by preventing close coupling of the target with the sample to be analyzed, and by preventing the x-ray detectors from being positioned in their optimum location.
In view of the foregoing problems and shortcomings with existing x-ray devices, and AXTs in particular, it would be an advancement in the art to provide an AXT employing a cathode and anode arrangement that would substantially minimize heating of the window and surrounding structures and that would substantially foreclose formation of filament deposits on the window, all without compromising the performance of the AXT. Additionally, the AXT should employ a relatively compact geometry so as to facilitate close coupling of the AXT with the sample to be analyzed.